1. Field of the Invention
The present invention relates to a structure in which a printed board is carried by a metal chassis, and more particularly to a printed board carrying chassis analyzing system, a printed board carrying chassis analyzing method, a printed board carrying chassis structure, and a printed board carrying chassis analyzing program for providing a screw-fastened arrangement which is effective to predict unnecessary radiation frequencies and reduce unnecessary radiation.
2. Description of the Related Art
As electronic devices have become higher in speed, the problem of unnecessary radiation (hereinafter referred to as EMI (Electro-Magnetic Interference)) from the chassis that carries printed boards thereon has become more noticeable. When a printed board is mounted on a chassis, it is the usual practice to electrically connect the ground plane of the printed board to the chassis with electrically conductive posts (hereinafter referred to as grounding posts) while keeping the ground plane spaced from the chassis by a distance ranging from several millimeters to several centimeters.
As pointed out in JP-A No. 2002-16391, for example, it has heretofore been known in the art that EMI from a chassis structure with a printed board mounted thereon increases when the printed board and the chassis surface confronting the printed board satisfy a particular resonating condition. However, no EMI predicting method has been established yet for a structure in which a grounding post is connected between a printed board and a chassis.
As disclosed in JP-A No. 2001-210922, it has been pointed out in the art that EMI can be reduced by grounding posts placed at the ends of the printed-wiring board and connected to ground patterns as outer frames of layers (signal layers, etc.) other than a ground layer. The disclosed structure is, however, disadvantageous in that the mounting area is relatively small because the ground patterns need to be placed as the outer frames of the signal layers.
JP-A No. 2004-158605 discloses a method of installing a printed-wiring board on a chassis. According to the disclosed method, resistive elements are inserted between power and ground layers of the printed-wiring board to reduce resonance of the power supply system to is thereby reduce the common-mode current flowing through the chassis. However, the disclosed method addresses the reduction of EMI from the power supply system, and is not concerned with the packaging technology for printed boards and chassis.
As described above, though the conventional chassis structure with the printed board mounted thereon can be analyzed for the prediction of the resonant frequency at which EMI is generated, if it has no grounding-posts, there is no method available to predict an EMI-generating resonant frequency for the conventional chassis structure if it has grounding posts. The conventional structure in which ground patterns are placed as the outer frames of the signal layers and are connected to the grounding posts that are connected between the printed-wiring board ends and the chassis is problematic in that the mounting area of the printed-wiring board is limited by the ground patterns added as the outer frames of the signal layers.